The present invention relates to tracking an object by optical means, and more specifically, to automatic monitoring and tracking a movable object such as an eye.
Monitoring and tracking a laterally movable object are important in many applications. In certain applications, it is desirable to have a tracking device not only to monitor the displacement of the object but also to follow the movement of the object without a significant delay. Tracking and following the eye movement during a laser eye surgery is an example of such applications.
Many eye-tracking devices have been developed for eye surgery with lasers, in particular, for photo-refractive surgery. A typical photo-refractive surgery scans an UV laser beam on the cornea to sculpture the profile of the corneal outer surface, one layer at a time. This procedure can correct various refractive disorders of the eye, including nearsightedness, farsightedness, and astigmatism.
Any eye movement during the surgery may adversely affect the outcome of refractive correction. Immobilizing the eye movement during a surgery has been proven difficult in practice. A device automatically tracking and compensating the eye movement is an attractive approach. For the nature of photo-refractive surgery, the tracking device needs to be fast, accurate, and reliable.
U.S. Pat. No. 5,620,436 discloses use of a video camera to monitor the eye""s movement and to determine the position of an aiming beam on the eye. U.S. Pat. No. 5,632,742 teaches projecting four laser spots on the eye and using a set of peak-and-hold circuits to determine the position of the eye. In these designs, a ring shape reference is used for determining the eye position, and spatial stationary infrared beams are applied to illuminate the reference. Sophisticated imaging system and electronics, such as a CCD camera or four peak-and-hold circuits are implemented to determine the position of the reference. The ring shape references are practically either the limbus or the iris of the eye and the whole ring is needed as the reference for determining the eye position.
Generally, any optically identifiable reference mark or indicator affix to an object can be used to indicate the position and movement of the object. The devices and methods disclosed herein apply an optical probe beam scanning repeatedly and rapidly over such a reference mark. A change in the position of the reference mark can then be determined by measuring the change in the delay between a predetermined reference time and the detected time at which the optical probe beam intercepts the reference mark. The reference mark can be artificially formed on the object, or alternatively, can be an inherent mark on the object.
For the application of eye tracking, a reference mark may be the limbus of the eye, which is the natural boundary between the transparent cornea and the white sclera. Optical scattering changes from one side of the limbus to the other significantly. Therefore the position of the limbus can be detected by measuring the timing of the change in the scattered light of the probe beam as the probe beam scans across the limbus. The devices and methods of the present disclosure will be described by examples of eye tracking using a section of the limbus as the reference mark.
In one embodiment, a section of the limbus is used as the reference mark and the x-y positions of the limbus are determined by two sets of linear positioning devices. The two linear positioning devices are set for measurement along two mutually orthogonal axes.
Each linear positioning device includes a scanning beam generator, a detection assembly, and a processing electronics. The scanning-beam generator projects an infrared probe beam onto the eye and scans the probe beam across a section of the limbus repetitively. The detection assembly detects the infrared light scattered from the eye. The detected scattered-light signal is a time-resolved signal and has a sequence of sharp steps corresponding to the probe beam repeatedly across the limbus. The timing of each sharp step depends on the limbus position at the corresponding scan. The processing electronics converts the timing of the sharp steps into the positioning signal indicating the position of the eye.
With the positioning signal, a system computer can then generate a control signal to steer the surgical laser beam to follow the movement of the eye. Hence, accurate laser surgery can be achieved even though the eye may move during the surgery.
In this embodiment, about a quart of the limbus is used to determine the x and y positions of the eye. This is particularly important for a new type of refractive surgery so called LASIK, in which part of the limbus is obstructed during the surgery. This embodiment can use the limbus section that is not blocked and thus it can use the limbus as a reliable reference mark for LASIK.